Terahertz radiation (THz) is a generic term for waves with a spectrum between 0.1 and 10 THz. Sometimes, the THz spectrum is defined as T-ray: especially in connection with imaging techniques. The frequency of 1 THz corresponds to a wavelength of 300 µm or 0.3 mm and to a wavenumber of 33 cm-1: THz fields have wavelengths extending from 3 mm (0,1 THz or 100 GHz) up to 30 µm (10 THz). This wavelength interval ranges between the top edge of the millimeter wave spectrum and the bottom edge of the optical spectrum, and corresponds to the boundary of the far-infrared (FIR) spectral region.
Short THz pulses most often are used for Fourier absorption and reflection spectroscopy. The absorption spectra of many polar molecules, for example H2O; C, N2;O2; O3; HCl, CO, SO2; CH3CN; etc., have many, distinct spectral peaks in the THz range. These unique signatures of the molecules in the THz range is of highest importance in monitoring the surrounding medium, air pollution detection, gas sensing, drugs and explosives detection.